Submission questions

I have never submitted poetry, but I'll offer my thoughts on submissions in general. To answer your first two questions, yes, you should include a cover letter. Check out the Writer's Market book (should be at your library and/or bookstore) -- it has examples of various types of query letters in the front.

Your third question isn't really cut and dry. I'd look at their writer's guidelines for an idea of how long you should wait. Sometimes Writer's Market will tell you if they accept simultaneous submissions (if they are in WM). I say go with your gut by considering how closely your poems match their publication. If there's a pretty good chance they'll accept, perhaps giving them some time before submitting elsewhere is a good idea.

If there is no mention of multiple or simultaneous submissions in thier guidelines assume they are not allowed because they are more than likely not. No simultaneous means you send your submission in and don't send it to anyone else until you get a rejection. No multiple means that you only send them one thing at a time.

Cover letters vary but in general a brief description of your work and maybe some biographical/past publishing details are not out of place. Generally you need to tell them how long it is (how many words) and what the broad genre/style is. Cover letters for magazines are not expected (in my experience) to be as long or involved as query letters or cover letters for novel submissions (where you need things like a full synopsis). If they don't ask for one or give no guidelines about what to put in it then stick to brief basics.